This invention relates to reduction of fatigue failure in a metal structural member, particularly in an area adjacent to an aperture, such as a drilled hole therein, and is especially concerned with the provision of novel stress coining apparatus and procedure for fatigue life extension of structural members.
A fatigue failure in a structural member is usually a gradual or progressive fracture resulting from fluctuating loads. The fracture starts at some point in the structural member at which the stress is much larger than anticipated. This high localized stress causes a small crack which then gradually separates as the flexing movement imparted to the structure is repeated, until the entire member ruptures without measurable yielding. It is well-known in this type of failure that the inclusion of an aperture therein, such as a drilled hole for the insertion of a fastening member, creates an area for the derivation of a high localized stress.
Thus, in a structural member having an aperture formed by any one of the conventional ways, as by drilling or reaming, a stress pattern normally develops in conjunction with the aperture wherein the lines of stress tend to converge toward the center of the aperture, so that as tensile stress is applied to the structural member, the magnitude of the force will be concentrated upon the aperture. In other words, the weakest portion of the member is in the area of the aperture.
A method and device for stress coining to increase fatigue strength and to minimize the stress concentration generated by an aperture in a metallic structure are disclosed in Applicant's U.S. Pat. Nos. 3,434,327 and 3,779,127. In Applicant's U.S. Pat. No. 3,951,561, there is disclosed a stress coining tool for increasing the fatigue strength of a metallic structural member having an aperture therein, comprising a rigid elongated shaft adapted for forcible insertion through the aperture, and provided with a plurality of different diameter, olive-shaped annular protuberances spaced intermediate the end portions, the protuberances being progressively larger in diameter from one end of the shaft. The largest of the protuberances has a diameter of such a relation to the diameter of the aperture that upon a forced insertion thereof through the aperture, the material forming the wall of the aperture is displaced radially substantially the difference between the diameter of the largest annular protuberance and the diameter of the aperture. The displaced material is thereafter allowed to rebound approximately one-half of the initial material displacement. As the result of such stress coining of the aperture, the lines of stress in the stress pattern of the member do not converge on the aperture and tend to flow around the aperture. Since the lines of force are not concentrated at any single point, the structural member functions substantially as a non-apertured member.
However, in using the prior art stress coining tools, it is often necessary to have access to the backside of the structural component through which the stress coining tool is passed. This limits the use of the tool since in many instances, the area adjacent the backside of the part is so limited as to prevent access thereto. Where there is insufficient access to the backside of the part, the stress coining tool cannot be pulled back through the stress coined aperture without further plastically expanding the hole and adversely affecting the stress coined characteristics of the structural member adjacent the aperture, wherein the lines of stress have been previously altered by the stress coining operation so that they do not converge on the aperture but tend to flow around the aperture.
It is accordingly an object of the present invention to provide an improved stress coining apparatus and procedure, which permits operation of the stress coining apparatus from only one side of a structural component.
Another object is the provision of a stress coining tool and procedure which permits insertion of the stress coining tool through an aperture on one side of the structural member, and pulling the tool back through the stress coined aperture, without adversely affecting the improved fatigue life characteristics of the initially stress coined aperture.
Yet another object of the invention is to provide means in conjuntion with an improved stress coining tool, and procedure, for vibrating tool, into and through an aperture in a structural member and then back through the resulting stress coined aperture, rapidly and without any elastic deformation of the stress coined aperture.